1. Technical Field
This invention is in the field of wheat (Triticum aestivum L.) breeding, specifically relating to wheat genotypes that are tolerant to the herbicide glyphosate.
2. Background Information
Weed competition is a primary cause of yield quality losses in wheat production. Jointed goatgrass, cheat grass and wild oats are major weed problems in wheat production systems in the Pacific Northwest (PNW), and direct seed production is completely reliant on chemical weed control. Most herbicides used to control these weeds are expensive and highly toxic. Yield losses from drought, Rhizoctonia root rot and weed competition range from 0% to nearly 100% depending on environmental conditions and the production system used. Developing varieties with resistance or tolerance to any one of these problems will greatly reduce economic risk factors associated with wheat production. Currently Rhizoctonia is managed by using glyphosate to eliminate infected plants from the previous year to control the green bridge effect, which typically occurs when fungal pathogens growing on roots of dying weeds and volunteer crops transfer to the roots of emerging cereal crops (Veseth, “‘Green Bridge’ Key to Root Disease Control,” PNW Conservation Tillage Handbook Series No. 16, chap. 4, “Disease Control,” pp. 1-8, 1992) The “greenbridge effect” phenomenon often results in significant plant stunting, reduced tillering and grain yield losses (Smiley and Wilkins, Plant Dis. 76:399-404, 1992; Hornby et al., “Take-all and Cereal Production Systems,” in: Take-all Disease of Cereals, Cambridge, U.K.: CAB International, pp. 103-164, 1998). With the removal of Roundup Ready® wheat (Monsanto Company, St. Louis, Mo.) from the commercialization process due to market acceptability concerns, herbicide-tolerant, transgenic wheat will not be available for many years, if ever.
Weed competition is a primary threat to commercial wheat production, resulting in decreased grain yields and inferior grain quality. Although cultivation can be used to eliminate weeds, soil from tilled fields is highly vulnerable to wind and water erosion. Due to ease of application and effectiveness, herbicide treatment is the preferred method of weed control. Herbicides also permit weed control in reduced tillage or direct seeded cropping systems designed to leave high levels of residue on the soil surface to prevent erosion. The most significant weed competition in wheat comes from highly related grasses, such as wild oat and jointed goatgrass. Unfortunately, it is difficult to devise effective chemical control strategies for problematic weed species related to the cultivated crop since they tend to share herbicide sensitivities. One approach to solving this problem involves the use of recombinant gene transfer to generate crop resistance to broad spectrum herbicides such as glyphosate (i.e. Roundup®) via genetic modification (GM), i.e., through the introduction of foreign gene sequences into plants through recombinant DNA and plant transformation techniques. In this system, herbicide is applied “in-crop” to control weeds without injuring the herbicide-tolerant crop plants. This approach was used to develop Roundup Ready® soybean, cotton, corn and canola varieties, which have been tremendously successful in the U.S. Roundup Ready® soybeans became available for commercial production in 1997, and by 2006, 71 of 75 million acres (95%) of soybeans grown in the U.S. were sown to Roundup Ready® varieties demonstrating the tremendous value of this technology (World Wide Web at nass.usda.gov). Producers credit higher net profits, an expanded herbicide application window, enhanced crop safety, and reduced soil erosion due to the elimination of tillage as the primary reasons for the wide-spread acceptance of Roundup Ready® soybeans.
In 1997, the Monsanto Corp. initiated collaborative efforts with private breeding companies and universities across the U.S. to develop Roundup Ready® spring wheat. Since other GM crops were already in commercial production, Roundup Ready® wheat was expected to be readily accepted. However, consumer perception of GM technology in wheat differed dramatically from other crops since wheat is primarily used for human consumption instead of animal feed; therefore, developing GM wheat was highly controversial. Based on economic impact assessments, investigators concluded that commercialization of GM wheat could result in the loss of 30 to 50% of U.S. export markets (Wisner, Economics Staff Report, Iowa State University Dept. of Economics, Ames, Iowa, 2004). Lack of consumer acceptance, particularly in Europe and Asia, eventually led industry representatives, including millers, bakers, and farmer organizations, to ban the production of GM wheat in the U.S. As a result, Monsanto halted the Roundup Ready® wheat development program in May of 2004, eliminating the possibility of using this approach to control problematic weeds in commercial wheat fields.
Alternative methods for developing herbicide-tolerant crop plants are available that do not involve genetic modification per se. Mutation breeding is a non-GM approach involving the use of chemical mutagenesis to increase genetic diversity for traits of agronomic value in crop plants. The process involves exposing seeds to a chemical mutagen, which generates changes in the DNA sequence of the plant resulting in the creation of novel, potentially useful genes that are transmitted from the original mutated plant (M1) to its offspring (M2) through normal sexual reproduction. Useful genes generated through mutation breeding are incorporated into adapted varieties using traditional cross-hybridization techniques. Chemical-induced variants are not considered to be GM since transformation (i.e. genetic engineering) is not used to insert the desired gene into the DNA of the host plant. The herbicide-tolerant Clearfield® Wheat, which is tolerant to Imidazolinone (Immi) herbicides, is the best known example of a wheat variety generated through mutation breeding. See U.S. Pat. No. 6,339,184. The tolerance gene was initially identified in a chemically-induced mutant derived from a French winter wheat variety (Newhouse et al., Plant Physiol. 100:882-886, 1992), and was subsequently transferred into other varieties through traditional breeding. The first Immi-tolerant winter wheat varieties went into commercial production in Colorado in 2003, and Clearfield® varieties are now available in every major winter wheat production region in the U.S. (Wide Web at nass.usda.gov). ORCF101, a Clearfield® variety released by Oregon State University, accounted for 6% of the soft white winter wheat acreage in Washington State in 2006, and acreage of Clearfield® varieties is expected to steadily increase over the next several years. Grain produced from Clearfield® varieties is non-regulated; therefore, it is sold as a bulk commodity without identity preservation or labeling requirements. Mutation breeding has also been used successfully to develop wheat varieties with resistance to powdery mildew (Kinane and Jones, Euphytica 117:251-260, 2001) leaf rust and stem rust (Williams et al., Crop Science 32:612-617, 1992, Friebe et al., Crop Science 34:400-404, 1994, Kerber and Aung, Crop Science 35:743-744, 1995), and yellow and brown rust.
U.S. Pat. No. 7,087,809 describes obtaining glyphosate-tolerant wheat that is tolerant to glyphosate by soaking non-mutagenized wheat seeds in a glyphosate solution and selecting plants that are glyphosate-tolerant.
The well-known “Roundup Ready®” gene used to make glyphosate tolerant soybean and maize by a GM approach is the result of a mutation in a bacterial gene encoding the enzyme target of glyphosate, EPSP synthase (Dill, Pest Manag. Sci. 61:219-224, 2005). Naturally occurring mutations in one or two genes have imparted glyphosate resistance to weed populations in areas where glyphosate was heavily used (Zelaya et al., Theor. Appl. Genet. 110:58-70, 2004; Owen and Zelaya, Pest Manag. Sci. 61:301-311, 2005). In addition, PCR mutagenesis of the cloned rice EPSP synthase gene showed that a single point mutation (C317T, P106L; that is, a single nucleotide change from cytosine to thymidine at nucleotide 317 resulting in an amino acid change in the EPSP protein from proline to lysine at amino acid 106) imparted glyphosate tolerance when transformed into and expressed in resulting transgenic plants (Zhou et al., Plant Physiol. 140:184-195, 2006). This proline codon is conserved in wheat EPSP synthase. Nonetheless, a majority of scientists in the field has held the opinion that a GM approach for developing glyphosate-tolerant crops was preferable since mutations induced by ethyl methane sulfonate (EMS) resulting in glyphosate-tolerant plants had not been identified to date in any plant species (Jander et al., Plant Physiol. 131:139-146, 2003; Dill, Pest Manag. Sci. 61:219-224, 2005). A screen of 125,000 mutagenized Arabidopsis plants failed to recover a single glyphosate-tolerant plant (Jander et al., Plant Physiol. 131:139-146, 2003). The authors suggested, “It is likely that no single-base change induced by EMS can produce glyphosate resistance in Arabidopsis.”
There is a need for new wheat varieties that are glyphosate-tolerant but that do not contain foreign DNA introduced into the plant genome by recombinant DNA techniques. The present invention meets these and other needs.